One major problem in wireless communication is how to obtain the greatest possible total rate of data transmission on the forward link, also referred to as the downlink, from a base station to its associated users, such as mobile stations, sharing a common frequency channel. Techniques for solving this problem must, among other things, address the interference that arises when the signal information destined for one user is received as an intermixture with signal information that was destined for other users.
Analogous problems occur in the context, e.g., of wireline networks. For example, it often happens that a plurality of electrical cables, each carrying a respective signal, fan out from an enclosure to a plurality of destinations. Within the enclosure, the signals are transported for some distance through an array of electrical conductors coupled closely enough to cause crosstalk. In such environments, the problem of how to maximize the total data-transmission rate must likewise address the problem of interference. Similar problems also arise in multimode optical fiber networks when signals are transmitted to multiple destinations on a common fiber at a common wavelength.
The techniques that provide the background for the present invention have been described mostly in the context of wireless communication. Accordingly, the below discussion will focus on techniques related to wireless communication. However, as will be appreciated by those skilled in the art, similar techniques are readily extended to, e.g., the wireline or optical context.
As is well known, in the absence of interference, a signal x transmitted from one base station antenna to one user will be received as received signal y=hx+w, wherein h is a complex number referred to as a “channel coefficient,” and w represents additive receiver noise. When an array of multiple base-station antennas transmits a respective signal to each of a plurality of users, each user receives a sum of received signals. Excluding additive receiver noise, each component of such a sum consists of the signal transmitted by one of the base-station antennas, weighted by the channel coefficient from the transmitting antenna to the receiver. As will be readily understood by those skilled in the art, the relationship between all transmitted and all received signals is concisely represented by the matrix equation:y=Hx+w,in which y is a vector of received signals, each component of which relates to a particular user, x is a vector of transmitted signals, each of which relates to a particular transmit antenna at the base station, H is a matrix of channel coefficients, and w is a vector of additive receiver noise, each component of which relates to a particular user.
One known method for removing inter-user interference from the received signals is to precede the signal vector x, before transmission, by multiplying it by the inverse of the channel matrix H. In other words, the transmitted signal vector x is replaced by the preceded vector xpc=H−1x. (Strictly speaking, this is valid only when the number of users equals the number of transmit antennas. There are known extensions to other cases.) As a consequence, excluding noise, and assuming that the number of transmit antennas equals the number of users, each user now receives only the signal from one respective transmit antenna. It should be understood that this scenario presupposes that the base station obtains measurements of the channel coefficients, and that these coefficients remain stable over one channel use, i.e., over the time taken to concurrently transmit one complex scalar signal value from each of the transmit antennas.
Although useful, the above-described method of preceding with channel inversion also suffers some disadvantages. For example, theoretical analysis of this preceding method predicts that when the channel coefficients are randomly distributed (more specifically, complex-Gaussian distributed with zero mean and unit variance) and the total transmit power is fixed, increasing the number of transmit antennas beyond a single antenna does not substantially increase the total rate of data transmission. This prediction is significant because practical experience has shown that the statistical model on which it is based is at least qualitatively accurate for rich scattering environments.
The performance of the preceded signal can be improved somewhat by applying a technique referred to as “regularizing” the inversion of the channel matrix. In accordance with regularized channel inversion the preceded signal vector is represented by the formulaxrpc=H*(HH*+αI)−1x,where H* is the complex transpose of the matrix H, α is a selected scalar value, and I is the unit matrix of dimension equal to the number of users. It should be noted that in this formulation, the number of transmit antennas is not required to equal the number of users. However, this formulation is particularly advantageous when the number of transmit antennas equals the number of users in communication with the base station at a given time.
Although useful, preceding by regularized channel inversion also suffers some disadvantages. Although some gain in total transmission rate is achieved, this gain remains significantly smaller than that which is theoretically possible to achieve when the number of transmit antennas is increased. Moreover, although the term α I in the above equation tends to reduce excessive demands for transmit power caused by outlying values of channel coefficients, the same term also tends to cause crosstalk among the received signals.
Thus, there remains a need for signal-conditioning methods that more fully achieve the gains that are theoretically possible in total transmission rate when multiple antennas are used at a base station or other source for distributing multiple signals to multiple users.